The present invention relates to a double pipe exhaust manifold which is interposed between an engine and a catalyst and in which an adiabatic outer pipe is disposed around an inner pipe through which exhaust gas passes in a state that a mesh spacer member is interposed between those pipes, whereby an adiabatic space is formed between the inner pipe and the outer pipe.
It is a common practice that the exhaust manifold, as shown in FIG. 6, has a double pipe structure including an inner pipe 101 and an adiabatic outer pipe 102 covering the outer periphery of the inner pipe, in order that a catalyst located in the midway of the exhaust system of an engine early exercises its purifying function by rapidly heating up the catalyst to facilitate the purifying performance of the vehicle by utilizing the heat of the exhaust gas. A mesh spacer member 103 is interposed between the inner pipe 101 and the outer pipe 102 to secure the adiabatic space. Since the mesh spacer member 103 is brought into contact with the inner pipe 101 and the outer pipe 102, a mesh consisting of wires each having a small diameter of about 0.25 mm is used for the mesh spacer member 103 so as to minimize its thermal conduction.
To give the inner pipe 101 a function of absorbing a thermal expansion difference between the inner pipe 101 and the outer pipe 102, which results from a thermal expansion difference and a thermal expansion coefficient between the inner pipe and the outer pipe, the inner pipe 101 consists of two pipe members coupled so as to allow those members to axially extend and shrink. The mesh spacer member 103 is fixed only to the inner pipe 101, and the outer pipe 102 and the mesh spacer member 103 are coupled such that those are slidable in the axial direction.
The outer pipe 102 is divided into two pipe members in the radial direction in the light of the assembling of the outer pipe 102 to the inner pipe 101. To assemble the outer pipe to the inner pipe 101, the divided outer pipe members 102a and 102b are both brought into contact with the outer periphery of the mesh spacer member 103 outside the inner pipe 101. In this state, one side end of the divided outer pipe member 102a is put on the corresponding side end of the divided outer pipe member 102b. The other side end of the former is also put on the corresponding one of the latter. Those overlapping portions of the divided outer pipe members 102a and 102b are bonded, by welding 104, into one cylindrical member. In this way, the outer pipe is assembled to the inner pipe 101.
In the conventional double pipe exhaust manifold, as described above, in a state that both the divided outer pipe members 102a and 102b are brought into contact with the outer peripheral surface of the mesh spacer member 103, those overlapping portions of the outer pipes 102a and 102b are bonded together by the welding 104. When the overlapping portions are welded together, a back bead 104a of the welding 104 comes in contact with the mesh spacer member 103. In this condition, the mesh spacer member 103 formed with fine wires of 0.25 mm in diameter is cut by high heat of the back bead 104a. As a result, there is the possibility that the mesh of the mesh spacer member starts to be broken from its cut part, and is loosened. The back bead 104a may be welded onto the mesh spacer member 103 although the mesh is not cut. In this case, the axially sliding motion of the outer pipe 102 to the inner pipe 101 will be impeded or break the mesh spacer member 103. The above problems may be solved in a manner that the outward flanges are formed at both side ends of the divided outer pipe members, and those flanges are welded together at the tips of them. In this approach, the outward flanges greatly project to the right and left from the outer pipe. Accordingly, the outside diameter of the exhaust manifold is increased by an amount corresponding to the flange projection. This results in deterioration of the on-board property.
It is an object of the present invention to provide a double pipe exhaust manifold which is able to prevent such an unwanted situation that during the assembling work by the welding of the divided outer pipe members forming the cylindrical outer pipe, the back bead comes in contact with the mesh spacer member, and the mesh spacer member is cut or the mesh spacer member is welded to the outer pipe by high heat of the back bead, without the deterioration of the on-board property.
The aforementioned object is achieved by means of a double pipe exhaust manifold having an inner pipe, a mesh spacer member applied to the outer periphery of the inner pipe, and an outer pipe being disposed around the outer periphery of the mesh spacer member in a state that the outer pipe is axially slidable to the inner pipe, wherein the outer pipe is divided into two pipe members in a radial direction, and one side end of one of the divided pipe members is put on the corresponding one of the other of the divided pipe members, and the other side end of the divided pipe member is put on the corresponding one of the latter divided pipe member in a radial direction, and the overlapping portions are welded together, the improvement being characterized in that the overlapping portions of the pipe members are swollen to the outside to form gaps between the overlapping portions and the mesh spacer member.
Preferably, the inner pipe is thinner than the outer pipe, and the mesh spacer member is fastened to the inner pipe by spot welding.
As described, in the invention, the overlapping portions of the pipe members are swollen to the outside to form gaps between the overlapping portions and the mesh spacer member. Accordingly, it is avoided that the back bead of the welding comes in contact with the mesh spacer member when one side end of one of the divided pipe members is put on the corresponding one of the other of the divided pipe members as radially viewed, and the other side end of the divided pipe member is put on the corresponding one of the latter divided pipe member, and in this state the overlapping portions are welded together.
The double pipe exhaust manifold of the invention successfully prevents such an unwanted situation that the back bead at high temperature comes in contact with the mesh spacer member, and hence the mesh spacer member is cut and the mesh spacer member is welded to the outer pipe.
It suffices that gaps between the overlapping portions and the mesh spacer member are minute (≈2 mm). Accordingly, there is no chance that the outside diameter of the exhaust manifold is increased and the on-board property is deteriorated.
In the preferred embodiment, the inner piper is thinner than the outer pipe, so that the mesh spacer member may be fastened to the inner pipe by spot welding. In the spot welding, temperature during the welding is lower than that in the cladding by welding. Therefore, the mesh spacer member may easily be fastened without the cutting of the mesh of the mesh spacer member.
Since the inner pipe is formed to have a thin thickness, a thermal capacity of it is small. Accordingly, it is prevented that heat is absorbed by the inner pipe and exhaust gas temperature reduces. Further, the outer pipe is formed to have a thick thickness, so that the durability of the double pipe exhaust manifold is increased.